While polymer blends have been reported in the literature from time to time, only within the past generation have polymer blends become sufficiently numerous and the advantages of polymer blends sufficiently apparent to warrant widespread investigation. While originally considered rare, miscible polymer blends more recently have been noted with increasing frequency.
Miscible blends of a second polymer and a first polymer have found utility in providing enhanced properties such as plasticization, tensile strength, melt processability, and increased resistance to heat distortion. Particularly in applications where polymeric materials are to be subjected to molding conditions, it is desirable that the modifying polymers blended into a polymer being molded be miscible in the first polymer; miscibility enhances the opportunity for avoiding weld-line strength difficulties and defects in finished, injection molded parts.
Likewise, blends characterized by immiscibility between polymer components of the blend have recently enjoyed increased attention as some such blends offer the potential for reduced melt viscosity and improved or desirable physical characteristics over the pure polymers. Where inclusion of a non-miscible second polymer as a minor constituent in a first polymer being processed can, for example, substantially reduce the temperature at which the first polymer can be processed, significant utility for such a blend may result. Likewise, where a blend of polymers, each substantially immiscible in the other, can enhance desirable solid-state physical characteristics or properties of the polymer in preponderance in the blend, substantial utility for the blend may result.
The prediction of miscibility between polymer pairs is still an art in infancy; miscibility is believed dependent upon a number of factors that include reactions between functional moieties pendant from one or more of the polymers, hydrogen bonding, and the like. Various suggestions have appeared for assisting in the selection of miscible or immiscible polymer pairs including an application of Flory's equation of state as set forth by L. P. McMaster, 6 Macromolecules, 760 (1973).
Other suggestions for useful tools in assessing misciblity of polymer pairs have included: two-dimensional solubility parameters, inverse gas chromatography; crystallization characteristics of polymer blends; and evaluation of glass transition temperature shifts, as suggested by L. M. Robeson, 24 Polymer Engineering and Science, p.p. 589 (June 1984). That the prediction of miscible polymer pairs is still an art, rather than a science, is indicated by, for example, by chlorinated polyethylene having 42 weight percent chlorine being miscible in poly(vinyl chloride), while chlorinated polyethylene havng a chlorine content less than 42% being immiscible in poly(vinyl chloride) as shown by Robeson, supra. at p.p. 588.
The prediction of properties of miscible and immiscible blends is also uncertain. While in some blend such desirable properties may follow simple additivity rules, other blends may show synergistic enhancement of desirable properties. Blends characterized by the components being immiscible typically are found to exhibit a tensile strength minimum while blends having components characterized by slight miscibility and a dual glass transition temperature may exhibit both a tensile minimum and a maximum as discussed by Fried, J. R., et al, 50 Journal of Applied Physics, p.p. 6052 (1979).
There is some significant indication that polymer blends tend to exhibit partial miscibility, and that graphical depictions of such partial miscibility tend to be of the minimum solution temperature type, that is those solubility curves having a minimum critical solution temperature below which a polymer pair exists in miscible state and above which, two phases are present, one phase being rich in a first polymer and the second phase being rich in a second polymer; Robeson, supra. p.p. 588. One possible explanation for a lack of solubility between polymer pairs may, in some cases, be related to the minimum critical solution temperature being lower than a glass transition temperature for one or both of the polymers; the polymers being below a glass transition temperature; a melt state for one or both of the polymers being non existent.
A number of substances forming blends with poly(vinyl chloride) have been identified in the literature, Robeson, supra. at p.p. 588, however, chlorinated poly(vinyl chloride) appears to have received less attention. Poly(vinyl chloride) and chlorinated poly(vinyl chloride) being chemically different compounds and particularly where properties of an immiscible blend of two polymers may be in part dependent upon such factors as hydrogen bonding and pendant functional moieties, the simple fact that poly(vinyl chloride) forms or does not form a desirable blend, whether miscible or immiscible, with a second polymer having desirable properties is not a particularly good indicator that chlorinated poly(vinyl chloride) will also form a blend with the second polymer having the desirable properties.
Poly(ethylene oxide) (PEO) blends in small proportions with another polymer have traditionally focused upon other such polymers as poly(acrylic acid) and poly(methacrylic acid) wherein strong complexing properties or miscibility with PEO characterizes the blend. Incompatible or immiscible polymer blends with PEO have attracted little interest in the literature except primarily for references to improved antistatic properties of some blends including PEO.
PEO has been mentioned as perhaps having utility in blends with poly(vinyl chloride) (PVC) as set forth in European Patent Application Al-0012559. PEO has been suggested for blending with chlorinated poly(vinyl chloride) (CPVC) in 128 Polymerization Kinetics and Technology p.p. 125-134, but the blend of CPVC and PEO discussed therein was characterized as leading "to a small improvement in impact strength and melt flow rate, but the heat distortion temperature was decreased."
While chlorinated poly(vinyl chloride) is possessed generally of performance characteristics and, particularly temperature performance characteristics, superior to poly(vinyl chloride), enhanced performance characteristics associated with a blend of chlorinated poly(vinyl chloride) and a second polymer together in an immiscible melt state could find substantial industrial utility.